As the capacity of the current cellular wireless networks is being reached, new frequency bands are introduced and respective wireless standards are being developed. 5G is one of those wireless standards where the majority of the newly introduced spectrum lies in the mmWave such as at 28, 38, or 66 GHz. Because of the steep attenuation characteristics at mmWave, mmWave 5G communication systems will most likely be line of sight (LOS), which will use phased arrays and direct the beam towards the base station/user equipment. As such, the power can be localized towards the receiver/transmitter and the power-noise figure requirements would be relieved on individual devices.
A phased array is essentially a group of antennas which have a same resonant frequency and are excited with a phase difference in between the adjacent elements that steer the beam to the desired direction. In recent years, patch antennas are gaining in popularity to form the phased array due to their low cost, easy fabrication process (may utilize conventional printed circuit board techniques in conjunction with other circuitry), and reasonable performance.
FIG. 1A shows a conventional patch antenna construction and probe-fed feeding scheme. A patch antenna 10 includes a patch 12, a substrate 14, a ground plane 16, a feed probe 18, and a feedline 20. The patch 12 is formed on a top surface of the substrate 14, while the ground plane 16 is formed on a bottom surface of the substrate 14. The feedline 20 is at the bottom surface of the substrate 14 (separate from the ground plane 16) and coupled to the patch 12 through the feed probe 18. An inner point of the patch 12, to which the feed probe 18 is touched and a radio frequency (RF) signal is provided, is a feed point F, which determines the input impedance for the patch antenna 10.
FIG. 1B shows a top view of the patch 12 with current distribution. The patch 12 has a width W and a length L orthogonal to the width W. The feed point F is centered along the width W of the patch 12 and off-centered along the length L of the patch 12. The current on the patch 12 mostly flows along a dimension, along which the feed point F is off-centered. Herein, the current on the patch 12 flows along the length L of the patch 12. For a given size of the patch 12, Eq. 1 represents the frequency the patch antenna 10 will resonate and radiate.
                              f          R                ≈                  c                      2            ⁢            L            ⁢                                                            ɛ                  r                                ⁢                                  ɛ                  0                                ⁢                                  μ                  0                                                                                        (        1        )            where c is the speed of light, L is the length of the patch 12, ε0 and μ0 are the free space permittivity and permeability, respectively, and εr is the effective relative permittivity.
A major drawback of the patch antenna 10 is the limited bandwidth. Typically, the patch antenna 10 only resonates at one frequency, which is determined by its dimensions and the substrate 14, on which the patch antenna 10 resides. As such, to implement phased arrays with different resonant frequencies, different sets of patch antennas are required, which is significantly area consuming. Therefore, there is a need for an improved antenna design, which could utilize the advantages of the patch antennas and provide tunable resonant frequencies using a same hardware.